Fuel rods for nuclear reactors are manufactured with extreme accuracy to avoid the occurrence of defects in the cladding which surrounds the fuel and hence leakage of radioactive fission products out into the reactor water in the primary circuit of the reactor. A leakage occurring leads to the reactor water and hence the different parts of the primary circuit being contaminated with the radioactive fission products. When a contamination of the reactor water has been determined or is suspected to have occurred, it is of the utmost importance that the leakage is located so that leaky fuel units can be replaced. In practice, leak detection is not normally carried out on individual fuel rods but on fuel assemblies containing several fuel rods. Also, when replacing fuel units it is fuel assemblies that are replaced and not individual fuel rods.
Leak detection of a suspected fuel assembly takes place by measuring fission products in a gas and/or water sample which is taken from the fuel assembly. To obtain as high a detection degree as possible, measures are therefore taken which lead to pumping of fission products out of the examined fuel assembly. This can be achieved by raising the temperature of the fuel or by reducing the external static pressure around the fuel. The temperature can be raised by stopping the water circulation through the fuel assembly, possibly by taking measures so that the water is completely removed from the fuel assembly. The external static pressure can be reduced by lifting the fuel assembly to a higher level in the reactor water.